Few data were available for the understanding of olfactory function in neuromyelitis optica spectrum disorders (NMOSDs). The aims of our study were to investigate the incidence of olfactory dysfunction and characterize olfactory structures, using MRI, in patients with NMOSDs. Olfactory function was evaluated by olfactometer in 49 patients with NMOSDs and 26 matched healthy controls. MRI parameters such as olfactory bulb (OB) and the olfactory-related gray matter volume changes were assessed. The frequency of olfactory dysfunction was 53 % in patients with NMOSDs. Olfactory detection thresholds were positively correlated with serum aquaporin-4 antibodies (fluorescent units) tested by fluorescent immunoprecipitation assay (FIPA) in NMOSDs (p = 0.009). Patients with olfactory dysfunction had smaller OB volume than did patients without olfactory dysfunction or controls (p < 0.01). Both detection and recognition thresholds for olfaction were negatively correlated with OB volume (p = 0.018, p < 0.01). The significant gray matter volume reduction in NMOSDs was found in the bilateral piriform cortex, orbitofrontal cortex, and parahippocampal gyri (FDR correction, p < 0.05, cluster size > 200 voxels). Our data suggested that olfactory function deficits are prevalent in patients with NMOSDs. Reduced OB and olfactory-related cortex volume may be responsible for the olfactory dysfunction.